Volumetric control for blower filter devices

ABSTRACT

The invention relates to a volumetric control for blower filter devices in which a control unit ( 5 ) determines a differential pressure between measuring points ( 1, 2 ) that is converted into a control signal for the fan output. To this end, at least two measuring points ( 1, 2 ) are arranged in the airflow behind the fan impeller ( 3 ) and in front of the consumer, in particular, the breathing hood ( 4 ). The measuring points ( 1, 2 ) can be located in the airflow inside the case filter device behind the impeller wheel ( 3 ) and in front of the outlet of the blower filter device or one measuring point is placed in the airflow inside the housing of the case filter device behind the impeller wheel ( 3 ) and one is placed in the vicinity of the connection of the breathing hood ( 4 ) or both measuring points are located in the breathing hose ( 8 ).

[0001] This invention relates to a volumetric control system for blower filter devices that is particularly suited for a breathing hood connection.

[0002] State-of-the-art blower filter devices are characterized by the disadvantage that the flow of air supplied to the hood varies depending on how clogged the filter is. When the filter is new and clean, more air passes through the filter as is required in accordance with applicable standards in a given individual case. Similar problems occur when different filters are to be used.

[0003] The resulting disadvantages are increased power consumption and increased air throughput. Another problem arising with the gradual clogging of the filter is that it is not known when the flow of air supplied to the hood falls below the required quantity. Another problem is that the type of connection that is used for the breathing hood influences the volumetric airflow.

[0004] Various types of volumetric controls have been designed to remedy this problem. EP 0 35 29 38 A2 proposes to measure the differential pressure between a measuring point in front of, and a measuring point behind, the impeller wheel of the fan and to use this signal for controlling the blower speed.

[0005] EP 0 62 10 56 A1 proposes to measure the dynamic pressure at the outlet of the blower filter device. The dynamic pressure is produced by the flow resistance of the hood and can also be used as a measure of volumetric airflow. In addition, this design features another sensor of the thermistor type in a side duct that monitors preset volumetric airflow limits and triggers an alarm signal when the airflow drops below these limits.

[0006] FI 80606 describes a design in which the fan motor is used as a detector so that the electrical control circuit measures the power drawn by the fan motor and the effective voltage at its poles. The design uses the properties of the rotary blower, as the air volume that flows through the blower per time unit is proportional to the rotor torque, and the pressure difference is proportional to the rotational speed. This solution is improved by DE 195 02 360 A1 in that the fan output is controlled based on current and rotational speed. Despite this comprehensive development effort, no one as yet has succeeded in keeping the volumetric airflow constant regardless of the filters and hoods that are used. Dynamic pressure measurement behind the fan or negative pressure measurement behind the fan can only be used to measure volumetric flow if the flow resistance values of the hood or filters are known. This means for practical purposes that the flow resistance values of filters and hoods have to be kept constant at narrow tolerances during production in order for these methods to work.

[0007] It is the problem of this invention to keep the volumetric flow constant within tolerance ranges regardless of the filters and hoods used. This problem is solved by the characterizing features of claim 1 while advantageous embodiments are the subject of the dependent claims.

[0008] According to the invention, a control unit controls the volumetric flow of blower filter devices by determining a differential pressure between measuring points and converting it into a control signal while at least two measuring points are arranged in the airflow behind the fan impeller wheel and in front of the consumer, in particular, the breathing hood. A number of tests have proven that the pressure difference in this measuring arrangement depends on volumetric airflow but is largely independent of the flow resistance of the filter(s) and the connection of the breathing hood.

[0009] In a preferred embodiment, the measuring points are positioned in the airflow within the case filter device behind the impeller wheel and in front of the outlet of the blower filter device. The pressure sensors and control equipment with power supply can thus be integrated in an optimum way into a compact unit with the blower filter device. Alternatively, one measuring point can be positioned behind the impeller wheel and another measuring point in front of the connection of the breathing hood in the breathing hose, or both measuring points can be placed in the breathing hose. It is always an advantage when the spacing of the two measuring points within the airflow portions described is as wide as is technically feasible.

[0010] The control unit compares the pressure difference with preset limiting values. If the pressure difference is outside preset limiting values, the control unit tries to set the volumetric airflow to the desired level (such as 125 l/min to 140 l/min) by changing the fan output. If this cannot be done, a signaling device is activated that alerts the user. This can be arranged by linking a measuring system with the fan in such a way that the signaling device is activated whenever the fan output exceeds or falls below limits, or by linking the signaling device with the control unit in such a way that the signaling device is activated when the differential pressure exceeds or falls below a preset differential pressure.

[0011] The figure shows an embodiment of the invention. The volumetric control for blower filter devices consists of the measuring points 1, 2 that are located in the airflow within the case filter device behind the impeller wheel 3 and in front of the blower filter outlet towards the breathing hose 8. Pressure sensors are placed at the measuring points 1, 2, and a control unit 5 determines the differential pressure between them and converts it into a control signal for the fan output. A signaling device 6 is activated when the volumetric airflow cannot be adjusted to a desired level in this way.

[0012] With this measuring point arrangement, the pressure difference depends on the volumetric airflow but is largely independent of the flow resistance of the filter(s) 7 and the breathing connection for the breathing hood 4. In this way, the volumetric airflow can be kept constant within tolerance ranges regardless of the filters 7 and breathing connections for breathing hoods 4 used.

[0013] List of Reference Symbols

[0014]1 measuring point

[0015]2 measuring point

[0016]3 impeller wheel

[0017]4 breathing hood

[0018]5 control unit

[0019]6 signaling device

[0020]7 filter

[0021]8 breathing hose 

We claim:
 1. A blower filter device with a volumetric control system comprising a fan with an impeller wheel (3), a filter (7) upstream of said impeller wheel (3), and a blower filter output, a control unit (5) operated by the differential pressure between two measuring points being connected to said fan, and said blower filter output being connected to a breathing hood (4) via a breathing hose (8), characterized in that at least two measuring points (1, 2) are arranged in the airflow behind the fan impeller wheel (3) and in front of the breathing hood (4) at the greatest technically feasible distance and in that a signaling device (6) is linked to the control unit (5) connected to measuring points (1, 2) that is activated when the measured values deviate from a specified differential pressure and/or a specified fan output.
 2. The blower filter device according to claim 1, characterized in that the measuring points (1, 2) are located behind the fan impeller wheel and in front of the blower filter output.
 3. The blower filter device according to claim 1, characterized in that the measuring points (1, 2) are located behind the fan impeller wheel and in front of the connection of the breathing hood.
 4. The blower filter device according to claim 1, characterized in that the measuring points (1, 2) are located in the breathing hose. 